QUICKLINKS

The Discovery of Neptune

16 August, 2009

Computer generated montage shows Neptune as it would appear from a spacecraft approaching Triton, Neptune's largest moon.Source: NASA/JPL/USGS

Question: Who discovered the planet Neptune?

Answer: In 1845, the English astronomer John Couch Adams had been investigating deviations of the planet Uranus from its regular orbit. He suspected that this "wobble" was being caused by an undiscovered planet lying outside the orbit of Uranus.

By an amazing coincidence and quite independently, in the same year, the French astronomer Urbain Jean Joseph Le Verrier was also investigating the irregularities in Uranus' orbit. Le Verrier arrived at the same conclusion and calculated where the planet might be found.

On September 23, 1846, the astronomers Johann Galle and Heinrich d'Arrest discovered Neptune just a few minutes into their search. Although they were the first to observe Neptune, credit for the planet's discovery is given to Adams and Le Verrier for their calculations that predicted where the planet would be.

However, it's even possible that Neptune's discovery is much older than we realise. Hidden in the notebooks of Galileo, the first astronomer to ever point a telescope at the sky, are observations that show Galileo unwittingly observed Neptune 234 years before its official discovery. Was Galileo aware that he'd seen a new planet? An investigation is underway to show that it just might be.